1. Field of the Invention
The present invention relates to a method for measuring a distance between reflection points present at an end face of an optical element and/or at a connecting point of optical fibers along a light transmission line used for optical communication and/or a reflectance thereof.
2. Related Art of the Invention
Along a light transmission line used for an optical communication system, the light may be reflected on a connecting surface or an end face of an optical element and at a connecting point of optical fibers. If there are a plurality of such reflection points, multiple reflection of signal light may be caused between the reflection points. Consequently, in addition to direct light having been transmitted through the reflection point, delay light caused by multiple reflection enters an optical receiver. In this way, when a multipath is produced along the light transmission line, the problem as described below will be developed.
It is known that transmitting analog signal light causes an increase in intermodulation distortion and harmonic distortion after transmission. This is reported, for example, in:
Reference 1: J. H. Angenent et al., "Distortion of a multicarrier signal due to optical reflections," EC0C91&I00C, WeC8-4, pp. 569-571, 1991.
The increase in distortion caused by multipath on a light transmission line is attributable to the following circumstances.
Multipath causes two types of signal light including direct light and delay light to enter the light receiving circuit. Beats of these types of signal light are detected as electric signals by the light receiving circuit. This beat is the distortion caused by multipath.
When transmitting digital signal light, the eye aperture ratio of the received signal is reduced by the delay light.
In order to improve performance of the system, it is necessary to inhibit production of the above-mentioned multipath. A point of occurrence of multipath can be predicted to some extent by measuring the distance between reflection points and reflectance of multiple reflection. Transmission performance can be improved by finding the point of occurrence of multipath in question and correcting it. It is therefore very important to measure the distance between reflection points and reflectance.
A conventional method for measuring the distance between reflection points and reflectance comprises the steps of using a semiconductor laser having low-distortion characteristics as a light source, modulating the light intensity of signal light by means of a sine wave signal for transmission, and detecting the then harmonic distortion or intermodulation distortion. This method will be described in some details below.
Assume that there are two reflection points along a light transmission line, with respective reflectance values of R.sub.1 and R.sub.2. If the distance between the reflection points is L, the k-degree harmonic distortion caused by multipath upon entering a carrier signal having a frequency of f.sub.1 into the semiconductor laser is approximately determined by the following formula: EQU HMD(kf.sub.1)=R.sub.1 R.sub.2 .xi.(2/m).sup.2 .multidot.sin.sup.2 (2.pi..nu..tau.+.theta..sub.k).multidot.{J.sub.k (B.sub.1)}.sup.2 (k=2,3, . . . ) (1)
The three-degree intermodulation distortion at a frequency of 2f.sub.1 .+-.f.sub.2 upon entering two carrier signals having respective frequencies of f.sub.1 and f.sub.2 into the semiconductor laser is approximately determined by the following formulae, where the two carrier signals have the same amplitude values: EQU IM3(2f.sub.1 .+-.f.sub.2)=R.sub.1 R.sub.2 .xi.(4/m).sup.2 .multidot.sin.sup.2 (2.pi..nu..tau.+.theta..sub.12).multidot.{J.sub.2 (B.sub.1)J.sub.1 (B.sub.2)}.sup.2 ( 2)
where, EQU B.sub.j =2{(.DELTA.F/.DELTA.I)I.sub.bt m/f.sub.j } sin (.pi.f.sub.j .tau.)(j=1,2) (3) EQU .tau.=2L/c (4)
where, .xi. is a polarization coupling coefficient of direct light and delay light; m is a modulation factor; J.sub.k (B.sub.j) is a k-degree Bessel function using B.sub.j as a variable; .nu. is a light frequency; .theta..sub.k and .theta..sub.12 are phase differences of direct light and delay light; (.DELTA.F/.DELTA.I) is a chirp amount of frequency of the transmitted light per unit current for the semiconductor laser; I.sub.bt is the difference between bias current and threshold current in the semiconductor laser; .tau. is the time difference between direct light and delay light; and c is the light velocity through the light transmission line. Both HMD in Formula (1) and IM3 in Formula (2) are normalized amplitude values of the carrier signal.
As shown in the calculation Formulae (1) of HMD and (2) of IM3, distortion caused by multipath periodically varies with the light frequency .nu.. FIG. 5 schematically illustrates characteristics of HMD relative to the light frequency .nu.. If HMD varies at a period .DELTA..nu., the distance between reflection points L can be expressed as c/(2.DELTA..nu.). To change the light frequency .nu., it suffices to alter temperature of the semi-conductor laser as the light source.
The product of multiplication of the two values of reflectance R.sub.1 R.sub.2 can be calculated from the maximum value of HMD in FIG. 5. If any one of the reflectance values R.sub.1 and R.sub.2 is known, it is possible to know the other.
It is thus possible to calculate reflectance of a reflection point on a light transmission line and the distance between reflection points from a harmonic distortion or an intermodulation distortion.
The problems involved in the above-mentioned conventional measuring method are as follows:
Problem 1:
When measuring multipath of light by the use of a value of harmonic distortion, measurement becomes more difficult the higher the frequency of the carrier signal becomes. A light receiver must measure two-degree or even three-degree harmonic distortion of the carrier signal. This requires a wider-band photo detector element and an electric amplifier of a higher frequency.
Problem 2:
When measuring multipath of light by the use of a value of intermodulation distortion, there are many parameters for experiment. The value of intermodulation distortion is, as is clear from Formula (2), largely dependent upon the frequency of two carrier signals and the degree of modulation thereof. When calculating reflectance of a reflection point, it is necessary to use detailed parameter values. It is therefore difficult to simplify measurement.
Problem 3:
When there is only a single delay light, the distance between reflection points can be easily derived, because the value of distortion shown in Formulae (1) and (2) periodically varies. When there are three or more reflection points and two or more delay lights, however, the distortion characteristics vary in a complex manner with the light frequency .nu., eliminating periodicity, and this makes it impossible to determine individual distances between reflection points.